1. Field of the Invention
The present invention relates to a digital watermark extracting method for appropriately extracting a digital watermark embedded in a document original or the like, an extracting apparatus, a program for executing such method, and a storage medium storing such program.
2. Related Background Art
Digital watermarking is attracting attention for copyright protection in circulating digitized data of image or sound on the Internet. The digital watermarking is a technology of processing digitized data of image or sound thereby embedding information in indiscernible manner to human. In the digital watermarking of a multi-value image, there are known various methods utilizing redundancy in pixel density.
On the other hand, so-called binary image such as a document original has little redundancy and the digital watermarking technology is difficult to apply.
However some methods for embedding digital watermark utilizing features specific to the document original are already known. For example, there are known a method of moving a base line of a row (U.S. Pat. No. 6,086,706), a method of manipulating a blank length between words (U.S. Pat. No. 5,861,619), a method of manipulating a blank length between letters (“Electronic document data hiding technique using inter-character space,” King Mongkut University, 1998 IEEE Asia-Pacific Conf. on Circuits and Systems, 1998, pp.419–422) and a method of handing as a black-and-white binary map (Japanese Patent Application Laid-open No. 11-234502).
In case a digital watermark is embedded in so-called binary image by the aforementioned digital watermark embedding method, then the image is printed, for example, by a printer and read by a scanner thereby being changed into a gray scale image, it is difficult to correctly extract the digital watermark from such gray scale image. The binarization of an image is usually conducted with a single fixed threshold value, but, for correct extraction of the digital watermark, another threshold value may be more appropriate.